CLHS: Issue FUNCTION-COMPOSITION Writeup

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## Issue FUNCTION-COMPOSITION Writeup

    Status: Passed (as amended) Jan 89 X3J13
    (And reaffirmed Jun 89 X3J13 that this was the version wanted)

    Forum:          Cleanup
    Issue:          FUNCTION-COMPOSITION
    References:     None
    Category:       ADDITION
    Edit history:   21-Jun-88, Version 1 by Pitman
                    05-Oct-88, Version 2 by Pitman
                     7-Dec-88, Version 3 by Masinter
                    12-Dec-88, Version 4 by Masinter (additional comments)
    		10-Feb-89, Version 5 (as amended by X3J13 Jan 89)
    Related-Issues: TEST-NOT-IF-NOT

    Problem Description:

      A number of useful functions on functions are conspicuously
      absent from Common Lisp's basic set. Among them are functions
      which return constant T, constant NIL, and functions which
      combine functions in common, interesting ways.

    Proposal (FUNCTION-COMPOSITION:JAN89-X3J13):

      Add the following functions:

       COMPLEMENT function					[Function]

        Returns a function whose value is the same as the NOT of the
        given function applied to the same arguments.

       CONSTANTLY value						[Function]

        Returns a function whose value is always VALUE.

    Examples:

      (MAPCAR #'(LAMBDA (X) (DECLARE (IGNORE X)) T) '(3 A 4.3))
      ==
      (MAPCAR (CONSTANTLY T) '(3 A 4.3))
      => (T T T)

      (FIND-IF-NOT #'ZEROP '(0 0 3))
      ==
      (FIND-IF (COMPLEMENT #'ZEROP) '(0 0 3))
      => 3

    Rationale:

      The presence of these functions will contribute to syntactic
      conciseness in some cases.

    Current Practice:

      No Common Lisp implementations provide these functions,
      but they do exist in the T language.

    Cost to Implementors:

      A straightforward implementation is simple to cook up. The definitions
      given here would suffice. Typically some additional work might be
      desirable to make these open code in interesting ways.

      (DEFUN COMPLEMENT (FUNCTION)
        #'(LAMBDA (&REST ARGUMENTS)
            (NOT (APPLY FUNCTION ARGUMENTS))))

      (DEFUN CONSTANTLY (VALUE)
        #'(LAMBDA (&REST ARGUMENTS)
            (DECLARE (IGNORE ARGUMENTS))
            VALUE))

    Cost to Users:

      None. This change is upward compatible.

    Cost of Non-Adoption:

      (COMPLEMENT BENEFITS)

    Benefits:

      Some code would be more clear.
      Some compilers might be able to produce better code.

      Takes a step toward being able to flush the -IF-NOT functions
      and the :TEST-NOT keywords, both of which are high on the list
      of what people are referring to when they say Common Lisp is
      bloated by too much garbage.

    Aesthetics:

      In situations where these could be used straightforwardly, the
      alternatives are far less perspicuous.

    Discussion:

      Several additional functions (COMPOSE, CONJOIN) were
     considered and rejected at the Jan 89 X3J13 meeting.

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